The cryopreservation of cells has been in practice for many years (Shannon and Macy, in Tissue Culture Methods and Applications (Eds. Kruse and Patterson) Academic Press, New York (1973) pp. 712-718). The use of europium-labeled target tumor cells in an assay of natural killer cell cytotoxicity activity has been previously described by Blomberg et al. (J. Immunoloqical Methods (1986) 86:225-229; J. Immunoloqical Methods (1986) 92:117-123; Granberg et al. J. Immunoloqical Methods (1988) 114:191-195). Cytotoxicity assays are used to test whether cells or drugs or biological agents (e.g., toxins, complement, lytic proteins, microorganisms and viruses) are toxic to target cells. For example, cytotoxicity assays are used to evaluate the activity of lymphocytes for adoptive immunotherapy of cancer (Rosenberg, U.S. Pat. No. 4,690,915 (1987)).